The present invention is directed towards a potentiometer and, more particularly, to a novel abrasion-resistant screen-printed potentiometer, and a novel method for making the same.
In conventional systems, potentiometers are mounted as individual components on a circuitboard. That is, the potentiometer is formed as a separate component and thereafter mounted on the circuitboard. As a result, two separate hand operations are required, increasing the overall cost of the resultant circuit.
In order to reduce manual operation and thereby reduce cost, potentiometers are often screen-printed directly on the circuitboard substrate. The major problem faced when manufacturing such devices is the need to make a low-cost integral potentiometer assembly which can withstand the abrasion effect of the wiper arm over several hundred thousand cycles. To this end, the prior art screen-printed potentiometers normally use a hard-surface material for the resistive element and a relatively soft material for the wiper assembly. By spring-loading the wiper with a high degree of compliance, a large amount of wear in the wiper contact can be accommodated. The drawback of this technique is that the a wiper having a soft, highly-compliant contact assembly is of relatively high cost. High quality potentiometers which use this technique are often two to three times more expensive than comparable component potentiometers.
In an alternate technique, the resistive component of the potentiometer is constructed of a relatively soft material and the wipers are constructed of a relatively hard material, such as graphite. Graphite has a tendency to be compacted into the surface of the resistive element, thereby substantially reducing the resistance across the surface of the resistive element and changing the net resistance of the potentiometer.
In another alternative technique, the potentiometer assembly is formed using a relatively hard metallic wiper and a relatively hard-surface resistive material. In potentiometers formed by this technique. the resistive material is worn away, thereby reducing the resistance in the area of wiper travel. Additionally, the use of materials having nearly equal hardness results in a potentiometer having noisy characters.